Introduction

How deep metamorphic rocks extrude is important to interpret the progressive deformation history of the convergent orogens. By sequential foreland propagating thrusting, these orogens accommodate crustal shortening of around a few 100 km (Panian and Pilant 1990). Newer thrusts of same dip direction develop towards the foreland and extrude rock masses towards the later at non-uniform intervals of several million years (Yin 2006). At least four additional regional structures/deformations may complicate this simple scenario viz. (1) foreland verging out-of-sequence thrusts that activate inside the previously thrust-up rock mass; (2) hinterland verging back thrusting; and (3) backslip of the interface between the two plates (Butler 1987; Morley 1988; Wang 2007; Larson et al. 2010; Catlos et al. 2007; Mukherjee et al. 2012 but many others). Additionally, around the late phase of collision for the large hot orogens, low-viscosity partially molten material may extrude in a single- or multiple pulses through one of the ‘shear zones’ (‘channels’) (e.g. Beaumont et al. 2001; Brown and Gibson 2006; Hollister and Grujic 2006; Rivers 2009; Mukherjee 2012a; Mukherjee and Koyi 2010a, b; Mukherjee et al. 2012), along with intermittent critical taper mechanism (Chambers et al. 2011; Larson et al. 2011) leading to a significant viscous dissipation (Mukherjee 2012a). Alternately, critical taper mechanism drove the channel flow (Long et al. 2012). Thermobarometric studies from central Bhutan Himalaya led Corrie et al. (2012) to propose that the Higher Himalaya underwent an ‘orogenic flattening’, which is a combination of a stronger critical taper and a weaker channel flow mechanism. Notice that neither channel flow (reviewed in Appendix-1 in Mukherjee 2012b, c) nor critical taper alone (as mentioned in Johnson and Harley 2012) can explain all the tectonic constrains of the Higher Himalaya.

Given its structural and metamorphic characters, the Higher Himalaya (Fig. 1)—one of the intra-continental longitudinal tectonic units of the Himalayan arc—has gained paramount attention for its exhumation mechanism (latest review by Mukherjee 2012b). Note that a zone bound by two shear zones can also lead to exhumation of high pressure rocks (e.g. Malavieille et al. 1998; Mukherjee and Mulchrone 2012). This work presents field-based structural geology of the Higher Himalaya Crystalline (HHC) from the Bhagirathi river section, NW Himalaya, India. Based mainly on the new findings of two ductile extensional sheared sub-zones and the brittle shear senses, the major deformation phases of the HHC is speculated. The shear sense indicators are usually of decimetre scale, whereas the identified brittle faults can be traced in near vertical sections.

Fig. 1
figure 1

The Higher Himalayan Crystalline (HHC) (from the MCTL up to the Martoli Fault). 1 Lesser Himalayan sedimentary rocks; 2a granite gneiss, mica schist; 2b mylonitized augen gneiss, mica schist; 2c phyllonite, chlorite schist; 3a sill/ky/gt/bt/mus schist/gneiss, migmatite; 3b augen Gneiss; 4 Bhaironghati Biotite Gneiss; 5 Gangotri Granite; 6 Tethyan Himalaya; MCT L Main Central Thrust-Lower. Reproduced from Jain et al. (2002). In this work, blue lines demarcate the South Tibetan Detachment System (STDS) and green lines the “basal detachment”. MCT U Main Central Thrust-Upper. The Jhala Normal Fault is traced as per fig. 2 of Catlos et al. (2002)

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Geology and tectonics

The Precambrian and Proterozoic HHC rocks along with Ordovician orthogneiss and Miocene leucogranites are dominantly gneisses and schists metamorphosed to greenschist to amphibolite facies (Yin 2006) and consist of patches of granulites and metabasites (Zhang et al. 2010’s review). The HHC could either be differently metamorphosed equivalents of the underlying Lesser- and the overlying Tethyan Himalayan units, or is a mélange of several crustal units with the Greater India during an early Paleozoic tectonics, that subsequently underwent extrusion by channel flow initiating from the mid-crustal rocks of south Tibet and/or by wedge mechanism (review: Gehrels et al. 2011).

The HHC is bound by the MCT-zone at the bottom at south and the South Tibetan Detachment System (STDS) as the top at north (Fig. 1). The upper (i.e. the northern) portion of the HHC migmatized due to extrusion of voluminous granitic melt coeval to the ductile extensional shear within the STDS. Throughout the HHC, a top-to-S/SW ductile shear from ~25 Ma took place. Limited geochronologic data indicate that the interior of the HHC was sheared between ~25–18 Ma (Carosi et al. 2013; Iaccarino et al. 2013). The 25 Ma timing nearly the same as that for the ‘hard collision’ between the Indian lithosphere and the Eurasian plate (van Hinsbergen et al. 2012). A late-phase ductile extensional shear within 19–14 Ma has been documented from the STDS (review by Godin et al. 2006). Inside the HHC, a ductile extensional shear during 24–12 Ma has also been reported from the ‘South Tibetan Detachment System-Lower’ inside the HHC, which is present rather discontinuously along the Himalayan trend (e.g. Carosi et al. 1998; Law et al. 2004; review by Godin et al. 2006). To avoid confusion with the original usage of the term ‘South Tibetan Detachment’, the extensional shear zone at the bottom of the HHC henceforth is described as the ‘basal detachment’. The part of the HHC not included inside the STDS and the basal detachment is top-to-S/SW sheared and may not be undeformed. Also regionally discontinuous is the geochronologically established out-of-sequence thrust plane inside the HHC active from 22 Ma till the Holocone, which is devoid of any unique structures across it (review by Mukherjee et al. 2012). The HHC, especially the Sutlej river in Himachal Pradesh (India), has revealed inverted metamorphism (Vannay et al. 2004). The MCT-zone has also revealed inverted metamorphism (from Nepal: Lombardo et al. 1993; Goscombe et al. 2006). Metamorphism within the HHC was either during the Eo-Himalayan (>44–33 Ma) and the Neo-Himalayan (~mid-Miocene) periods (reviewed as fig. 7a in Streule et al. 2010) or as a single phase (review by Yakymchuk and Godin 2012). A regional kyanite to sillimanite-grade metamorphism during 32-20 Ma took place within the HHC (St-Onge et al. 2006). The HHC could have undergone a HP metamorphism at a depth of ~80–100 km (Yang et al. 2011), but not many supports this.

The HHC in the Bhagirathi section in particular consists of garnet-biotite schists and gneisses, metadolerites, phyllonites, actinolite schists, carbonates, calc-silicates, talc-tremolite schist, amphibolites, migmatites and granites (Metcalfe 1993; Singh and Thakur 2001; Srivastava and Tripathi 2007; Jain et al. 2002; Tripathi et al. 2009; Sachan et al. 2010). The Bhagirathi Granite (/Badrinath Granite/Gangotri Granite)—located at the northern portion of the HHC—has been described as en-echelon lensoid intrusions consisting of potassium feldspar, quartz, plagioclase, tourmaline, muscovite, biotite and garnet (Searle et al. 1993) that might had formed from a Pan-African rock body (Singh et al. 2003). This granite body exhumed at a much faster rate initiating from ~2.4 Ma with a concomitant denudation at a rate of ~2 mm year−1 (Sorkhabi et al. 1996). For a much broader span during 21–5 Ma, the middle of the HHC in the Garhwal Himalaya exhumed at a rate of ~0.67 ± 0.13 mm year−1 (Yin 2006). Linero (2008) deduced a near constant exhumation rate of the HHC in the Bhagirathi section of 1.43 mm year−1 since last ~20 Ma. Scaillet et al. (1995) surmised that the granite body was emplaced during extensional shear of the upper boundary of the HHC (also see Searle et al. 1993). Jain et al. (2002), from a review of previous geochronologic data available from the Bhagirathi Granite, considered 22–23 Ma as its most plausible cooling age and 3 mm year−1 its exhumation rate. The cooling rate of the Higher Himalayan granite in the Garhwal Himalaya drastically reduced from ~175–300 up to ~16–20 °C km−1 from 23-01 Ma (Searle et al. 1999).

Around 10 km thick MCT-zone bound by the MCT (=MCTL of Godin et al. 2006 = Munsiari Thrust as referred in Catlos et al. 2002 = MCT2 as followed by Mitra et al. 2010) at south, and the Vaikrita Thrust (=MCTU of Godin et al. 2006 = MCT1 as followed by Mitra et al. 2010; also see Khannal and Robinson 2012) at north in the Garhwal Himalaya has been studied in greater detail than its further northern portions in the Bhagirathi section. Based on 40Ar/39Ar plateau age of hornblende, Metcalfe (1993) deduced that the MCT in the Garhwal sector was active at ~19.8 ± 2.6 Ma. This broadly matches with the relatively brief span of 21–19 Ma of activation of the MCT-zone in this portion of the Himalaya (Searle et al. 1999). Catlos et al. (2002) obtained a much younger monazite age of 5.9 ± 0.2 Ma from the MCTU. The MCT-zone shows an increasing grade of metamorphism towards N starting from chlorite up to the kyanite grade. An increase in temperature from 500 to 770 °C was also deciphered in structural up-section across this zone (Metcalfe 1993). Extrusion took place from north to south under decreasing thermobarometric conditions (Sachan et al. 2001). Near the MCT, mylonitized quartzite, S-C fabrics, prominent stretching lineations of micas and feldspars, and recrystallization of grains were reported (Singh and Thakur 2001; Jain et al. 2002; Tripathi et al. 2009). Three phases of folding were documented from the entire HHC (Jain et al. 2002), and specifically from the MCT-zone (Srivastava and Tripathi 2007; Tripathi et al. 2009; also see Roy and Valdiya 1988)—but none of these deformations seem to be regional. This is supported by a near uniform N/NE dip of the ‘main foliations’ (or the primary shear C-planes of Passchier and Trouw 2005; e.g. Jain et al. 2002). A heterogeneous deformation led to ≥140–210 km displacement within the MCT-zone in the Garhwal region (Singh and Thakur 2001; Tripathi et al. 2009). Singh and Thakur (2001) deduced a strain ratio of 2.3:1.8:1 from the MCT-zone. Further, Tripathi and Srivastava (2005) deciphered shearing in conjugate sets, and a NNE–SSW compression simultaneous to the slip of the MCT (also see Srivastava et al. 2000). The middle of the MCT-zone underwent maximum strain and the magnitude fell down away from there in response to a non-coaxial shear (Srivastava et al. 2000; also see Grasemann et al. 1999 for another section). The MCT has been considered by some as a ‘defunct Miocene structure’ (Kirby and Whipple 2012). Although the component of pure shear has been deduced from a number of sections of the HHC (reviewed in fig. 8 of Mukherjee 2012b; also see Larson and Godin 2009), a similar quantification has not been done along the entire length of the Bhagirathi river within the HHC.

Mukherjee and Koyi (2010a) demarcated the STDS as a top-to-N/NE ductile extensional shear zone at the Sutlej section. The northern boundary of the STDS matched with the contact between the Higher and the Tethyan Himalaya and the southern boundary falls between the locations Pangi and Kashang. In contrary, the tectonics of the Bhagirathi section of the HHC is not yet well understood since the southern boundary of the STDS remained unmarked. Yin (2006) interpreted Metcalfe’s (1993) data of uniform cooling age of ~21 Ma across the Jhala fault inside the HHC as indicative of its inactivation during the Miocene Period of coeval compressional shear within the MCT and extensional shear of the STDS. In contrast to Sorkhabi et al.’s (1999) interpretation that it is purely a thrust, Catlos et al. (2002) and Yin (2006) considered that the Jhala Fault underwent a normal sense of movement in its late stage. Does the Jhala Fault demarcate the southern boundary of the STDS?

Structural geology

Ductile deformations

The HHC is top-to-S/SW ductile sheared. The primary shear C-planes dip towards N/NE. The spacing between the C-planes varies from ~1 to 15 cm. The shear is exemplified by S-C fabrics and usually sigmoid-shaped clasts (Berthé et al. 1979; Mukherjee and Bandyopadhyay 2011; Mukherjee 2011) of various morphologies (Figs. 2, 3). Some of the S-planes are defined by sigmoid quatzofeldspathic grains akin to sigma-structures (Fig. 2d), very rare delta (δ) structures (Fig. 2b), isolated curved grains of biotites (Fig. 2c), lenticular feldspar clasts with major axes tilted along the shear direction (Fig. 2d) etc. Delta structures indicate possibly a higher rate of rotation than the sigma (σ) structures (Passchier and Trouw 2005). Therefore, rare occurrence of the former structures indicates qualitatively a variation in the intensity of shear within the HHC. Also, different shapes of shear sense indicators occur side by side, such as the delta structures and individual grains of sigmoid shapes (Fig. 2d). At a few locations, the C-planes are sub-horizontal (Fig. 2a).

Fig. 2
figure 2

Top-to-SW ductile shear sense indicators from the HHC, Bhagirathi section. a Mylonite with sub-horizontal C-planes. Location-2; at Sainj (30°45′19.4″N; 78°34′31.4″E). b A delta structure. Adjacent to it is a sigmoid elongated quartz Location-4. (30°48′1.8″N; 78°37′00″E). c Top-to-SW ductile shear sense indicators from the HHC, Bhagirathi section. S-C fabric defined by biotites. Location—30°51′37″N; 78°39′2.6″E. d Sigmoid and lenticular feldspar clasts. Width of photo: 18 cm. Location-6 30°0′42″N; 78°42′25.6″E

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Fig. 3
figure 3

a, b Extensional top-to-NE ductile shear inside the basal detachment—from sigmoidal quartzofeldspathic mineral aggregates within mylonitized gneiss. a Width of photo: 14 cm. Location: Near the MCTL, Sainj village (30°46′14.2″N; 78°35′51.8″E)

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A top-to-N/NE extensional ductile shear confines within two sub-zones. The same set of N/NE dipping main foliations bound the S-fabrics. The extensional zone in south, demarcated by blue lines in Fig. 1, covers partly the MCT-zone and is designated as the ‘basal detachment’. Its upper boundary is close to the village Bhatwari and is ~20 km SW from the Vaikrita Thrust (=MCTU). Strongly asymmetric quartzofeldspathic grains of various thicknesses define this shear (Fig. 3a, b). Close-spaced (cm to mm scale) C-planes and at places S-fabrics sub-parallel to the C-planes connote intense shear (interpreted as per Davis and Reynolds 1996; Twiss and Moores 2007). These extensional sub-zones were deciphered based on structures and fabrics, usually of cm scale, on rocks that can be closely observed exposed on the metal road section. For rocks exposed far away and across the road, it was not possible to trace the shear fabrics. Thus, the observed boundaries of those two extensional sub-zones were located only on the road/river section. For the sake of presentation, those have been drawn as short lines. In reality, those moderately dip towards NE.

The green lines in Fig. 1 demarcate the ‘South Tibetan Detachment System-Upper’ (STDS) as the second extensional zone. It occurs near the northern/upper portion of the HHC. The shear senses are indicated mainly by sigma-like structures of asymmetric quartzofeldspathic aggregates and veins (Fig. 4a–c) and one rare delta-like structure (Fig. 4d). The upper boundary of the STDS is the Malari Fault. The southern boundary lies ~5 km SW to the location Jhala. Thus, the ‘Jhala Normal Fault’ is a part of the STDS and lies inside the latter. This is as per Searle et al.’s (1999) postulation but does not match with Catlos et al. (2002) who demarcated the Jhala Fault as the northern boundary of the Higher Himalaya by excluding the Gangotri Granite from the latter. Notice that except the upper boundary of the STDS, none of the other three boundaries of these two extensional zones coincide with any litho-contacts. This matches with the HHC in the Everest massif, Nepal Himalaya (Searle 1999) and Gonto La in southern Tibet (Edwards et al. 1996). In these locations, the basal detachment occurs much NE to the MCTL. However, it mismatches with the Sutlej section of the HHC where Mukherjee and Koyi (2010a) demarcated the basal detachment nearly coincident with the MCTU. Within the STDS and the basal detachment, fabrics of top-to-S/SW shear are less frequent than the top-to-N/NE ones. Therefore, following the standard explanation (Patel et al. 1993; Argles and Edwards 2002; Yin 2006; Mukherjee and Koyi 2010a, b), the S/SW shear might have preceded the N/NE one. Notice that an extensional ductile shear zone in the lower portion of the HHC in Nepal has also been referred by Sapkota and Sanislav (2012).

Fig. 4
figure 4

Top-to-NE sense of ductile shear within the STDS (near the upper portion of the HHC). a, b, d Sigma-like structure of quartz vein; photo widths: ~17 cm; c a delta-like structure. Photo width: ~40 cm. Locations: a 31°0′53.1′N, 78°42′29.4′E; b 31°01′56.6″N; 78°43′15.2″E; c 31°02′5.9″N; 78°43′31.8″E; d 31°01′18″N; 78°52′16.7″E

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No clear-cut secondary shear planes—C′ and C″ (Passchier and Trouw 2005) associate with the primary shear S-C fabrics for the top-to-S/SW and the top-to-N/NE shear. This possibly indicates insignificant pure shear component associated with that primary shear (interpretation as per Goscombe et al. 2006). Localized shear zones at low angles to the main foliation, however, occur exclusively within the pinch and swell structures (Fig. 5). Therefore, these might be due to local coaxial deformation that gave rise to boudins and do not have any tectonic implications (as per Schmalholz and Maeder 2012). Pinch and swell structures indicate non-Newtonian rheology of the matrix (Schmalholz and Maeder 2012). Therefore, modelling the HHC as a non-Newtonian fluid to explain its extrusion (Beaumont et al. 2001) is more appropriate than those based on its Newtonian rheology (such as Mukherjee 2012d). The geometry of scar folds varies considerably in these boudins. Where the matrix is non-foliated, these folds obviously did not develop. Variation in geometry of these folds could be explained by either a disparity in the intensity of shear or due to different viscosity contrasts between the clasts and the matrices (Maeder et al. 2009).

Fig. 5
figure 5

Boudins of quartz veins from the HHC. Local brittle ductile shear (orange half arrows) in a, b; a photo widths: 26 cm. Location: 30°48′1.8″N, 78°37.0″E. b Photo width: 21 cm. Location: 30°48′1.8″N, 78°37.0″E. c Photo width: 12 cm. Location: 30°54′18.1″N; 78°40′44.5″E

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Brittle deformations

A top-to-S/SW brittle shear took place throughout the HHC (Figs. 6, 7) bound by a number of Y-planes that dip ~towards NE. The brittle shear is revealed in terms of inclined brittle planes (the P-planes) of various curvatures bound by parallel sets of shear planes (the Y-planes) with cm (Fig. 6a) up to ~a metre spacing (Fig. 6c). The termination of a number of P-planes across the Y-plane is a well-known feature (such as fig. 6.23a in Bahat et al. 2005). Sometimes, instead of a well-developed pair (Fig. 6a, c), only a single Y-plane is found (Fig. 6b). A single brittle plane may be weavy that locally define the Y-plane, and elsewhere the P-plane (Fig. 6b). Boudinaged quartz clasts may subsequently thrust giving rise to sigmoid lenses (Fig. 6d). Along with distinct P-planes, the rock rarely resembles ‘hard ash’, which may be due to substantial shear heating (photographs with the author).

Fig. 6
figure 6

Top-to-SW brittle sense of shear defined by P-planes (blue full arrow) and Y-planes (green full arrow). b A pair of P-planes are curved and sub-parallel. Photo width: 18 cm. Location: 30°45′23.2″N; 78°34′37.8″E. d A boudinaged clast of quartz developed P-planes. Location: 30°50′3.5′/N; 78°37′22.9″E

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Fig. 7
figure 7

a–c Step-overs shown by blue half arrows, developed for a top-to-S/SW brittle shear- shown by green half arrow. c. Intersecting Y-plane of step-over forms a lens (white full arrow). The NE dipping fracture plane is pointed by a green arrow. d The Y-plane of the step-over forms gouge where brittle shear planes and sense developed. Photo width: 14 cm. Location: 31°01′00″N; 78°42′34.9″E

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Step-overs or ‘oversteps’ of top-to-S/SW brittle shear cut the pre-existing top-to-S/SW brittle shear bound within NE dipping Y-planes (Fig. 7). The Y-planes of oversteps are undulatory and usually are sub-horizontal to gently dipping. At places, they converge and diverge to produce nearly symmetric lenses (Fig. 7c). These lenses alone are not indicative of shear sense. The Y-planes at places are sharp and devoid of any gouge (Fig. 7a, b). At other places, P-planes developed inside gouge zones (Fig. 7d).

Interestingly, an additional top-to-N/NE sense of brittle shear as backthrusts was documented from the HHC (Figs. 8, 9, 10) except its northern portion of Gangotri Granite. Backthrusts occur here as a zone, with lengths of individual thrusts up to several metres. The distances amongst these backthrusts are up to about half a metre. Their Y-planes dip moderate to steeply towards S/SW and bound sigmoid to nearly straight P-planes. The Y-planes may be restricted as a particular zone inside the rock (Fig. 8a). There are observations that only a set of P-planes developed devoid of any Y-planes (Fig. 8b, c). Both the Y-planes and the P-planes cut across the NE dipping main foliation (particularly Fig. 8c, d). The P-planes developed at various extents—for example, few centimetre up to about few metres (Fig. 9b). Few of the P-planes are not so well developed (Fig. 10a). Individual P-planes curve sigmoidally inside brittle sheared sigmoid bulges (Fig. 10b). Shapes of bulges/lenses may not give any shear sense. Spacing between individual Y-planes may vary (Fig. 10c), Fore-thrusts (top-to-S/SW shear sense) and backthrusts (top-to-N/NE shear sense) sometimes constitute ‘triangular zones’.

Fig. 8
figure 8

Top-to-NE sense of brittle shear (blue half arrows) inside the HHC. Well-developed Y-planes in a and d but not in b and c. NE dipping main foliation (green line) is cut by the P-planes. Location: 31°51′50.6″N; 78°39′2.8″E

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Fig. 9
figure 9

Top-to-NE sense of brittle shear (blue half arrows) inside the HHC. a, c Indistinct Y-planes. b blue ellipse: a geologist as a marker. Locations: 31°51′37.7″N; 78°39′3.8″E

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Fig. 10
figure 10

Top-to-NE sense of brittle shear inside the HHC. That in c is of a much larger scale (white circle: a geologist as a marker). Location: 31°51′7.8″N; 78°38′22.3″E

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Brittle shear senses inside the Gangotri Granite are different and are less commonly developed than the remainder of the HHC. For example, the brittle shear is top-to-N/NE extensional (Fig. 11a), top-to-E compressional (Fig. 11b), top-to-S/SW extensional (Fig. 11c, d), top-to-N extensional (Fig. 12a), top-to-WSW compressional (Fig. 12b) and top-to-E compressional (Fig. 12c). These senses can be deciphered either in sub-vertical sections where the asymmetry of the P-planes is visible (Fig. 12a, c, d) or based on triangular peaks developed on the fault planes containing slickenlines (Fig. 11b). The Y-planes may or may not be developed in pairs (compare Fig. 12c with d) even for the same brittle shear sense. The spacing between the two adjacent P-planes changes along their length (Fig. 12a). The upper portion of the HHC, viz. the STDS, and most notably the vast exposure of the Gangotri Granite did not backthrust in a top-to-N/NE sense.

Fig. 11
figure 11

Diverse shear senses documented from the Gangotri Granite. a Top-to-NE extensional, at 31°02′36.6″N, 78°50′31.2″E. b Slickenslides with triangular apices pointing up-dip. Top-to-E compressional, at 31°02′6.8″N, 78°51′32.1″E. c Top-to-SW extensional, at 31°02′6.8″N, 78°51′32.1″E. d Top-to-SW extensional, at 31°1′47.8″N; 78°52′6″E

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Fig. 12
figure 12

Diverse shear senses documented from the Gangotri Granite. a Top-to-N extensional; at 31°01′27.7″N, 78°52′9.8″E. b Spacing between adjacent P-planes differ (at green and blue full arrows). Top-to-WSW compressional, at 31°01′27.7″N, 78°52′9.8″E. c Top-to-E compressional, at 31°01′24.8″N, 78°52′12.7″E

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Discussions

Detachments

A structural summary of the Bhagirathi section of the HHC is presented schematically in Fig. 13. Since the demarcated deformation shear zones inside the HHC were only surface observations, their extent and geometries below the surface remains speculative. The basal detachment developed inside the HHC that partly overlaps the MCT-Zone corroborates similar findings from other sections such as from the Langtang section in Nepal (Takagi et al. 2003), Sutlej valley (Vannay et al. 2004; Mukherjee and Koyi 2010a) in the western Himalaya and the Mangde Chu river section in Bhutan (Jain et al. 2012). While Vannay et al. (2004), in their designated ‘Karcham Normal Fault’, did not deduce any activation timings, Takagi et al. (2003) deciphered Pliocene–Pleistocene Periods as the possible range of activation. From these limited information, Yin (2006) proposed that possibly the MCT acted as an extensional shear zone during the Cenozoic. This is despite the magnitude and consequence(s) of slip remained unknown. Other than these, Guillot (1999) reviewed several cases of MCT activity during the post-Miocene time. Presence of an extensional shear zone inside the HHC has been reported from the central and the eastern Himalaya (Godin et al. 2006’s review). However, unlike those reported from the Nepalese Himalaya where it is a brittle extensional shear zone (Searle 1999a), the basal detachment in the Bhagirathi section is defined by a zone of top-to-N/NE ductile shear fabrics (Fig. 3). Presence of two ductile detachments from the Sutlej section was explained by Mukherjee and Koyi (2010a) as due to shifting mode of channel flow of the HHC (Fig. 14a). A shifting channel flow was also proposed by Hollister and Grujic (2006) who explained the out-of-sequence thrusting from the Bhutan Himalaya (Fig. 14b). In such shifting modes, one of the pulses of channel flow (extrusive pressure gradient plus shear at the boundaries) took place on the entire HHC. In the other pulse, channel flow (of the same meaning) was restricted at the southern sub-zone within the HHC. The northern boundary of this sub-zone is demarcated by the top of the basal detachment and the southern boundary the MCTL. Mukherjee et al. (2012) demonstrated that simultaneous development of two detachments inside the HHC is kinematically implausible given the parallel or diverging-up geometry of the boundaries of the HHC. For converging-up boundaries as proposed by Webb et al. (2007), two detachments probably cannot develop simultaneously. Thus, despite the absence of timing of extensional shear of the basal detachment, channel flow in shifting mode might gave rise to the two detachments. However, which detachment developed earlier remains unresolved. This is because although some estimate for the activation of the STDS by extensional ductile shear is already known (23–21 Ma: Searle et al. 1999; <21.9 Ma: Harrison et al. 1997, no movement of the Jhala Fault during ~21 Ma), that for the basal detachment is unavailable.

Fig. 13
figure 13

The dominant structural geology of the HHC, Bhagirathi section, is summarized in a NE–SW cross-section. HHC Higher Himalayan Crystalline; MCT L Main Central Thrust-Lower; LH Lesser Himalaya; MCT U Main Central Thrust-Upper; BD Basal Detachment; STDS South Tibetan Detachment System; TH Tethyan Himalaya; 1 top-to-NW ductile compressional shear; 2 top-to-NE ductile extensional shear; 3 Boudins; 4 top-to-SW brittle compressional shear; 5 top-to-NE brittle compressional shear (backthrust); 6 top-to-SW brittle extensional shear; 7 top-to-N extensional brittle shear. The diagram is neither to scale nor angle

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Fig. 14
figure 14

Shifting channel flow model proposed from the HHC. a Across the vertices of the parabolic flow profiles, ductile shear sense reverses. Lines parallel to the boundaries of the HHC passing through the vertex define the lower boundary of the detachment. Drawn from fig. 17 of Mukherjee and Koyi (2010a). Half arrows represent the ductile shear senses of sub-zones that they bound. b One of the flow pulses is bound by the OOST. Drawn from fig. 5 of Hollister and Grujic (2006). HHC Higher Himalayan Crystalline; STDS South Tibetan Detachment System; BD Basal Detachment; OOST Out-of-sequence thrust; MCT Main Central Thrust

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A number of geoscientific papers support channel flow of the HHC in the Garhwal Himalaya. The partially molten rock that Nelson et al. (1996) documented in southern Tibet at a mid-crustal depth continues presumably ~5 km below the MCT-zone (Israil et al. 2008; Ashish et al. 2009). Two phases of magmatism from the Bhagirathi section of the HHC at ~46 and ~20 Ma were interpreted as the pulses of channel flow (Jain et al. 2005).

Step-overs/oversteps

‘Oversteps’ (Ahlgren 2001), form lozenges (like fig. 13 of Fusseis et al. 2006) of a uniform asymmetry that gives the shear sense. Step-overs were described initially from strike slip fault zones (de Paola et al. 1996). Later, it seems to be common in other shear zones as well (e.g. Ponce et al. 2012). These authors explained the morphology of such lozenges from a shear zone in Cap de Creus area (eastern Pyrenees). They describe the lozenges, which are less deformed than the matrix, have no relation between their shapes to the strain and the deformation kinematics. Lozenges form due to intersection between the ductile primary (C) and the secondary shear planes (C′ and C″), but the detail mechanism is yet not clear (Ponce et al. 2010). The long axes of the lozenges make a low angle to the ‘mean trend of the deformation zone’ (i.e. the C-planes) at <30°. The lozenges were divided into four morphological categories: rhombic, lensoidal, rhomboidal and sigmoid (Ponce et al. 2012). In this study area, lensoidal and sigmoidal lenses are common. As in ductile shear zones (fig. 5 of Pereira and Silva 2004), the step-overs in the HHC developed in the brittle regime due to intersection of the primary brittle shear Y-planes and local synthetic Riedel shear (Fig. 7c). The Y-planes are low-dipping and weavy. The networking of top-to-S/SW brittle shear zones possibly indicates deformation partitioning during this shear (Fusseis et al. 2006).

Backthrusts

Backthrusts were noted from various orogens in the Earth, mainly in collisional orogens including acreetionary prisms. From the Himalaya in particular, backthrusts have been reported from the Siwalik and the Lesser Himalaya (Chalaron et al. 1995; Mugnier et al. 1998; Powers et al. 1998; Mugnier et al. 1999, Mukhopadhyay and Mishra 2005; Yin 2006 and references therein; Guha et al. 2007; Thakur et al. 2007; Malik et al. 2010; review by Kumarahara and Jayangondaperumal 2013). The ‘Great Counter Thrust’ (GCT/South Kailash Thrust System/Renbu-Zedang Thrust/Himalayan backthrust/Main Zanskar Backthrust)—that demarcates the boundary between the Tethyan sediments in S and the Asian plate in N—is a backthrust with N/NE vergence and dominantly a southward movement and folding of the hanging wall block (Heim and Gansser 1939; Yin et al. 1999). The GCT thrust system consists of five or more individual thrusts (Murphy and Yin 2003) that affect only the top portion of the crust, slipped at a rate of ~7 mm year−1 during 30–23 Ma (Harrison et al. 2000) and has a two-stage tectonic evolution (Yin et al. 1999) with a displacement of ≥12 km (Quidelleur et al. 1997). The GCT acted coevally with the Main Central Thrust (MCT) and the STDS at south and the backthrusts of the south central Tibet at ~mid-Miocene Period (Hébert et al. 2012). Based on U–Th/He dating, Tripathi (2011) obtained the timing of GCT between 19 and 7 Ma. This posed doubt whether GCT, MCT and STDS acted coevally and whether GCT can be considered as a single thrust system. It extends regionally from the western up to the eastern Himalaya. The Great Counter Thrust shows some strike slip evidences as well (Zhang et al. 2011). The thrust has been deciphered in sub-surface based on seismic anisotropic studies (Sherrington et al. 2004). In SE Tibet, the Great Counter Thrust oriented magnetic lineations a SSW trend (Antolín et al. 2011). Since the link between this thrust and the STDS has remained doubtful (Yin 2006), the existing tectonic models of the Higher Himalaya do not refer it. No backthrusts were reported from the Higher Himalaya from field before the present work. Thus, the observed backthrusts in this section is not regionally extensive along the Himalayan trend. Notice that very recently, however, the STDS has been proposed to be a backthrust (e.g. He et al. 2012; Kellett and Grujic 2012; Webb and He 2012).

The tendency of the viscous channel flow alone that initiates from south Tibet is to produce folds and ductile shear zones with a NE vergence inside the STDS and a SW vergence within the remainder of the HHC. Thus, backthrusting of the HHC cannot be explained to push on the brittle rocks by ductile extrusion from bottom. Instead, the critical taper model could be useful to explain them. Backthrusting becomes dominant near the bottom of the taper zones of the wedge, and they become equally frequent as the fore-thrusts for a low taper angle (Bilotti and Shaw 2005). Backthrusts develop the taper condition within the wedge (McQuarrie and DeCelles 2001).

Backthrusts may develop when low-frictional material passes from a flat into a ramp (Butler 1987; Maillot and Leroy 2003; Nieuwland et al. 2010). In the present case, a low friction between the channel boundaries and the extruding material is expected since the HHC during extrusion was in a partially molten state. Partial melting was achieved by shear heating: (1) over the MCT ramp (Harrison et al. 1997); (2) below south Tibet possibly due to crustal thickening and overpressure (review by Harris 2007); and (3) around the central zone of the HHC (Mukherjee 2012c). Alternately, the melt could have come from (4) the fluids released from the underlying Lesser Himalaya (Guo and Wilson 2011); and/or (5) melting of the HHC rocks in situ (e.g. the Haimanta-Cheka Formation in Nepal: Searle et al. 2010). Given these different sources, the resultant softening of the HHC is expected to be non-uniform yet significant.

Backthrusting can develop in collisional settings either due to reversal of polarity of subduction, due to an interference between varied isostatic uplift and horizontal compression in collisional regimes (review in Kroehler et al. 2011; Moores and Twiss 1995), or by deformation of several rock layers under compression (Albanese and Sulli 2012). Interestingly, Fielding (2000) presented in his crustal section that the Indian plate subducted below the Eurasian plate not just as a slab (discussed later) but penetrated the ductile lower crust of the Eurasian plate as a wedge (Fig. 15). Our textbook idea that the Indian plate subducted as a coherent unit below the Eurasian plate from the Late Jurassic, and the oceanic portion of the Eurasian plate subducted during Late Triassic to Earle Jurassic (e.g. fig. 10.15 in Keary et al.2009) got fine tuned with upcoming many seismic tomographic studies (review by Kumar 2012; fig. 4 of Guillot and Replumaz 2013; Replumaz et al. 2013). Zeng et al. (2007) deciphered from seismic anomalies documented from a section between Lhasa to Golmud (Tibet) that there could be several northward and southward subductions in the India-Eurasia collisional regime. Chen et al. (2010) deciphered that the Indian mantle lithosphere got detached from its crustal counterpart and moved significantly northward below the Eurasian plate. In other words, the Indian crust and the Indian lithospheric mantle did not act as a single unit (Replumaz et al. 2010). Liang et al. (2012) interpreted that the upper mantle beneath the Tibetan plateau is possibly subducting southward below the Indian plate. Flow of mantle towards south has been predicted to corroborate this from mathematical models (Zahirovic et al. 2012). Tomographic studies have deciphered shallow but significant southward subduction of the Eurasian plate in Pamir region (Negredo et al. 2007; also see Mechie et al. 2012; Schneider et al. 2012), which happened at a slower rate than that of the Indian plate (see Vignon et al. 2011), and probably initiated before ~21 Ma (Lukens et al. 2012). Robinson et al. (2012) deciphered southward subduction of the Asian oceanic crust during early Jurassic Period. This was subsequently supported by gravity studies by Tiwari et al. (2010). Even a flip in the subduction direction of the Indian plate from north to south has been reported (Koulakov and Sobolev 2006). Kumar et al. (2006) deciphered by seismic studies from Lhasa (Tibet) that the Eurasian plate there subducted up to ~175 km depth southward at a moderate angle. The Eurasian plate could have moved below the Indian plate in a sub-horizontal manner (Yue et al. 2012). Zhao et al. (2010) presented a more detailed scenario that the Asian mantle lithosphere remains sub-horizontal below Pamir and Karakoram and dips southward to reach ~250 km depth, whereas in the eastern Himalaya, along the ‘eastern line’, this subduction is more restricted (nearly the same transect what Kumar et al. 2006 worked earlier); no subduction has been deciphered in the central section of the orogen through the ‘central line’(Fig. 16; also see Kind et al. 2010 for a similar conclusion). A similar west to east fall in southward advance of the Eurasian mantle was also deciphered by Li et al. (2008). It thus appears that the Eurasian plate was not merely static withstanding compression of the Indian plate, but the former also actively pushed and moved southward. The zone in between the subducted mantle slabs of the Indian and the Eurasian plates could either be a ‘crush zone’ of rocks (Zhao et al. 2010) or a hot mantle diapir (Zhang et al. 2012). This supports Zhao et al.’s (2011) postulation that the lithospheric architecture in the India-Eurasia subduction zone is indeed complex.

Fig. 15
figure 15

Subduction of the Indian plate below Tibet took place as by wedging between it and the ductile lower crust of the Eurasian plate. Reproduced from fig. 10.11 of Fielding (2000). Geographical directions (N/NE, S/SW) are added

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Fig. 16
figure 16

Geophysical studies undertaken across the Himalayan trend, taken from fig. 1 of Zhao et al. (2010), is drawn on the Himalaya map reproduced from fig. 1 of .Godin et al. (2006). b Crustal section along those lines reproduced from fig. 4 of Zhao et al. (2010). Green Asian mantle lithosphere; red crush zone; yellow crust; blue Indian mantle lithosphere. MBT Main Boundary Thrust; MCT Main Central Thrust; BNS Bangong-Nujiang Suture; QBF Quaidam Basin Fault

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The present observation of numerous backthrusts documented in the field within the entire HHC from the Bhagirathi section locates nearly equidistant from the Tien Shan-Karakoram line and the West line along which the deep seismic images were presented by Zhao et al. (2010). The ‘West line’ of Zhao et al. (2010) is in close proximity with Lower Dolpo, Annapurna, Dhaulagiri (Nepal), and Dhauliganga and Goriganga sections (India) (shown in Fig. 16a). Therefore, further structural fieldworks in these sections might reveal backthrusts inside the HHC. The likelihood of getting backthrusts much away from this area seems unlikely except the Pamir region where southward subduction of both the Indian and the Eurasian plates have been deciphered (Koulakov and Sobolev 2006; Negredo et al. 2007). In several recent seismic studies, ~400 km subduction of the Indian plate has been deciphered. This could be due to a corner flow of the Indian asthenosphere stronger than that of the Eurasian asthenopshere (fig. 5 of Fu et al. 2008; fig. 9 of Yue et al. 2012).

Brittle shear

One of the theories of brittle shear is that the shear plane Y-forms first. This is followed by the P-planes at an angle to the Y-planes. The P-planes (~‘fault arrays’), usually sigmoid, originates from the Y-planes and migrate towards the nearest other Y-plane (Fusseis et al. 2006; see Fig. 17). In the HHC, this is evidenced by (1) the dominant NE dipping fractures ubiquitously developed inside the HHC (excepting the Gangotri Granite) and the Y-planes for the top-to-S/SW brittle shear have nearly same attitude and (2) sometimes P-planes of half-sigmoid shapes emanate from the NE dipping Y-planes. This also possibly indicates that the top-to-S/SW brittle shear acted along many of the pre-existing NE dipping fracture planes. These fractures locally acted as the Y-planes and developed brittle P-planes.

Fig. 17
figure 17

Evolution of brittle shear zones as presented by Tchalenko (1970) and summarized by Handy et al. (2007). After branching out from the Y-planes that migrate along their lengths, the P-planes tend to join sigmoidally with the adjacent Y-plane

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A number of brittle shear senses documented from the Gangotri Granite cannot be explained simply in terms of backthrusting towards the hinterland side (i.e. towards N/NE direction). Assuming that the HHC once attained a critical taper condition, at least the top-to-S/SW extensional brittle shear can readily be explained in terms of wedge tectonics of the HHC. Matching the push of the model wedge materials towards SW same as the direction of extrusion of the HHC, one can explain top-to-S/SW extensional shear. Further, backthrusts and out-of-sequence thrusts are the manifestations of wedge mechanics (Platt 1986). The observed backthrusts throughout the HHC support a wedge mechanism/critical taper model of extrusion which might have acted for the HHC.

Conclusions

In addition to a top-to-S/SW shear, two sub-zones (STDS, basal detachment) of top-to-N/NE shear are demarcated inside the Bhagirathi river section of the Higher Himalayan Crystalline (HHC). The upper boundary of the STDS also demarcates the top of the HHC. The lower margin of the former lies around the middle of the HHC. The extensional shear zone ‘Jhala Fault’ falls inside the STDS. The basal detachment occurs within the southern portion of the Main Central Thrust-Zone (MCTZ). Besides a top-to-S/SW brittle shear (fore thrusting), a series of top-to-N/NE backthrusts characterize the HHC except its northern portion occupies by the Gangotri Granite. The S/SW shear is characterized by step-overs that indicate strain localization. The Gangotri Granite displays a number of other extensional and compressional brittle shear senses. Two ductile sub-zones inside the HHC might be produced by shifting channel flow of the HHC. Pure shear leading selectively to the detachment inside in HHC (here the ‘basal detachment’) was considered to be tectonically implausible by Mukherjee and Koyi (2010a) from the Sutlej section of the Himalaya. Backthrusting from the HHC was hitherto not reported from any section of the Higher Himalaya. These backthrusts cannot be correlated with any out-of-sequence thrusting since such deformation has not been documented from the present study area in the Bhagirathi section. Significant subduction of the Eurasian plate below the Indian plate in the Garhwal Himalaya could develop these backthrusts. It appears that the top-to-S/SW brittle shear developed along Y-planes that previously acted merely as NE dipping fractures. The critical taper model can explain at least the top-to-S/SW extensional shear inside the Gangotri Granite. It thus appears that both channel flow and critical taper exhumation mechanisms acted inside the HHC. It might be possible that the hot partially molten mid-crustal material after being extruded to the brittle regime was under wedge tectonics. This is in accordance with the recent numerical models by Beaumont and Jamieson (2010), geochronologic findings by Chambers et al. (2011), Larson et al. (2011), Long et al. (2012) and thermobarometric works by Corrie et al. (2012) and Larson et al. (2013).

Caldwell et al. (2012) based on geophysical evidences postulated the presence of a ramp and flat geometry of the HHC in the Garhwal region (also reviewed by Rai and Ramesh 2012; Singh et al. 2012; see Gahalaut and Arora 2012). Change in Moho depth within the Higher Himalaya as deciphered from gravity studies from an eastern (Tiwari et al. 2010) and a western Himalayan section (Chamoli et al. 2011) could also be an indicator of ramp and flat geometry. Such a thrust geometry can favour a critical taper situation.